The continuing electronics revolution has intensified the demand for high energy density rechargeable batteries. To meet this demand, much research has been conducted on metal ion chemistries. Of particular interest in this regard are lithium ion batteries, which typically employ a carbon, metal oxide or metal chalcogenide anode, a lithium salt dissolved in a non-aqueous solvent as the electrolyte, and a lithium metal oxide as the cathode.
Metal ion batteries are usually constructed by separately forming an anode and a cathode, placing an electrolyte between the anode and cathode to form a battery, and then giving the battery an initial charge. Components in the electrolytes used in known metal-ion batteries are sometimes thermodynamically unstable at the electrode potentials employed, and the initial charging of such batteries results in breakdown of such components at the anode. This causes the so-called "first cycle capacity loss" or "irreversible capacity loss". The electrolyte breakdown attending the initial charging cycle occurs significantly at first, but is greatly reduced by the formation of a passivating surface film on the electrode. This passivating surface film is known in the industry as a solid electrolyte interface (SEI).
Formation of the SEI is both advantageous and disadvantageous. On the plus side, timely formation of an SEI reduces the first cycle capacity loss. In commercially available metal ion batteries, for example, first cycle capacity loss is generally limited to less than about 5-10%. In addition, an effective SEI is substantially impermeable to electrolyte, while still being relatively permeable to metal ions. This provides metal ion battery electrolytes with kinetic stability, and results in good cycle life.
On the down side, the need for development of an effective SEI has previously limited the choice of electrolytes. Many known electrolytes having desirable characteristics such as low volatility, high flash point, low freezing point, or high dielectric constant, for example, are unstable on the anodes and fail to produce an effective SEI. Consequently, such electrolytes have previously been used only in relatively low concentrations., (See, e.g., Sony EP 0 696 077, the text of which is incorporated herein by reference).
To a certain extent, development of an effective SEI also limits the choice of anodes. It is known, for example, that propylene carbonate is substantially incompatible with graphite anodes because the graphite catalyzes decomposition of the propylene carbonate, without producing an effective SEI. As a result, propylene carbonate has not heretofore been employed with graphite anodes. 12-Crown-4 has been used as an additive to propylene carbonate to minimize the amount of irreversible capacity during the first intercalation of lithium [see, e.g., "Lithium Batteries--New Materials, Developments and Perspectives", ed. G. Pistoia, Elsevier, 1994), however its use is undesirable because of its known toxicity. Thus, there is a considerable need to develop methods for the development of metal ion batteries that can use electrolytes that are substantially incompatible with the anode material being used.